The present invention relates to a combustor for burning fuel in compressed air. More specifically, the present invention relates to a combustor in which fuel is introduced by fuel tubes into two pre-mixing passages.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Additional air was introduced into the combustor downstream of the primary combustion zone. Although the overall fuel/air ratio was considerably less than stoichiometric, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range of firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of such approximately stoichiometric fuel/air mixtures resulted in very high temperatures in the primary combustion zone. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. It is known that combustion at lean fuel/air ratios reduces NOx formation. However, achieving such lean mixtures requires that the fuel be widely distributed and very well mixed into the combustion air. This can be accomplished by introducing the fuel into the combustion air in both primary and secondary annular passages so that the fuel and air are pre-mixed prior to their introduction into the combustion zones.
It has been found that mixing of the primary fuel and combustion air is enhanced by using two primary pre-mixing passages. Fuel is introduced into these passages by a number of fuel spray tubes, or "pegs," that extend through the two passages and that are distributed around their circumference. A combustor of this type is shown in U.S. Pat. No. 5,479,782 (Parker et al.), hereby incorporated by reference in its entirety.
Unfortunately, such fuel tubes do not allow the fuel introduced into the two primary pre-mixing passages to be individually regulated. This lack of control prevents optimization of the combustion dynamics.
It is therefore desirable to provide a combustor, such as that suitable for use in a gas turbine, in which the flow of fuel to multiple primary pre-mixing passages can be individually controlled.